Services for delivering video content such as movies over the Internet or the like have in use in recent years. Such video content is encrypted by Digital Rights Management (DRM) technology before delivery. DRM technology protects the video content against illegal copying and distribution. Notwithstanding such copy protection, there are many instances in which content viewers illegally copy video content by capturing the video content displayed on a computer display or a television monitor by using an image capturing apparatus such as a camcorder, and illegally distribute the thus copied video content. Since the video content displayed on the display is not encrypted, it is difficult to prevent the distribution of the video content copied by capturing the displayed video.
In view of the above, a technology has been developed that embeds information such as a viewer identification number into video data in the form of a digital watermark before delivering the video data. If the video data in which the digital watermark is embedded is illegally copied by capturing the displayed image using a camcorder, the digital watermark remains intact in the copied video data. Therefore, if the video data illegally copied by means of such analog capture is distributed, the identification number of the viewer that has made illegal use of the video content can be identified by detecting the digital watermark from the illegally copied video data. Accordingly, even when the video content illegally copied by means of analog capture is uploaded to a video uploading website, for example, the administrator of the video content can identify the illegal copy source from the information embedded in the uploaded video content.
Generally, it is preferable to embed a digital watermark in video data in such a manner that the digital watermark embedded in the video data is imperceptible to the viewer. To embed the digital watermark in the video data so as to be imperceptible to the viewer, it is preferable to reduce the variation in luminance that occurs due to the presence of the digital watermark, i.e., the difference in pixel value between the digital watermark embedded region and its peripheral region. However, if the difference in pixel value between the digital watermark embedded region and its peripheral region is small, the digital watermark may be lost when the video data in which the digital watermark is embedded is compressed using a video compression standard such as Moving Picture Experts Group (MPEG). The digital watermark may also be lost due to image distortions, etc., that occur when the video data in which the digital watermark is embedded is captured using a camcorder or the like. On the other hand, if the difference in pixel value between the digital watermark embedded region and its surrounding region is large, the above problem may not occur, but in this case, the digital watermark may become perceptible to the viewer. In other words, there is a tradeoff between the degree to which the pixel value can be made to spatially change due to the presence of the digital watermark and the degree to which the digital watermark can be made robust. To solve this tradeoff problem, various digital watermarking techniques have been invented (for example, refer to Japanese Laid-open Patent Publication No. H03-10483, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2007/102403, Japanese Laid-open Patent Publication No. 2004-179744, and Japanese Laid-open Patent Publication No. 2006-270215).
For example, Japanese Laid-open Patent Publication No. H03-10483 discloses an appended signal multiplexing apparatus in which a main signal is output in a field repetitive manner, an appended signal is also output in a field repetitive manner but by inverting its phase for each field, and the main signal and the appended signal are multiplexed together.
Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2007/102403 discloses a digital watermark embedding apparatus which embeds embedding information in the form of a digital watermark into an input signal having dimensions equal to or greater than N. This digital watermark embedding apparatus generates an (N−1)-dimensional pattern based on the embedding information, generates an N-dimensional embedding pattern by modulating a periodic signal in accordance with values on the (N−1)-dimensional pattern, and superimposes the embedding pattern on the input signal. Alternatively, the digital watermark embedding apparatus superimposes the (N−1)-dimensional pattern on an (N−1)-dimensional plane in a portion of the signal obtained by orthogonal transformation of the input signal, and applies an inverse orthogonal transformation to the signal on which the (N−1)-dimensional pattern has been superimposed.
Japanese Laid-open Patent Publication No. 2004-179744 discloses a digital watermark embedding apparatus in which the position of a geometric pattern embedded in a moving image is caused to vary along a time axis on a frame-by-frame basis in accordance with encoded watermark information. The digital watermark embedding apparatus causes the position of the geometric pattern to vary with time in such a manner as to be convex upward or downward depending on the value of the watermark information.
Japanese Laid-open Patent Publication No. 2006-270215 discloses a digital watermark embedding method in which a symbol component contained in a digital watermark pattern to be embedded in each frame is caused to vary from frame to frame, for example, using a spread pattern generated by spread spectrum.
According to the technique disclosed in Japanese Laid-open Patent Publication No. H03-10483, a digital watermark detection apparatus for detecting the embedded information enhances the embedded information by obtaining inter-frame differences and detects the thus enhanced information. This requires that the moving image frame displayed on the display be precisely synchronized with the frame of the video camera used to capture the video data presented on the display. However, it is quite rare that the frame display rate of the display precisely coincides with the frame capture rate of the video camera. As a result, it is highly likely that the digital watermark detection apparatus fails to detect the digital watermark information from the video data copied by means of analog capture.
The technique disclosed in Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2007/102403 differs from the technique disclosed in Japanese Laid-open Patent Publication No. H03-10483 in that the precise frame synchronization is not needed. Therefore, according to the technique disclosed in Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2007/102403, the robustness of the watermark information against analog capture is improved. However, the technique disclosed in Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2007/102403 has the problem that a phenomenon called “defocusing” that causes noise to appear stuck to an image occurs because the position of the embedded watermark is fixed.
On the other hand, according to the technique disclosed in Japanese Laid-open Patent Publication No. 2004-179744, if some of the frames are decimated from the video data created by analog-capturing the video data in which the digital watermark information is embedded, it becomes difficult to locate the reference position that is set midway between the upper and lower edges of the geometric pattern. This makes it difficult to detect the digital watermark. Further, in order to locate the position of the geometric pattern in the image, the difference in pixel value between the region where the geometric pattern is embedded and its peripheral region has to be made large enough, which results in image quality degradation. Furthermore, since the image generally contains many images of objects moving in translational fashion, it is difficult to distinguish the movements of such objects from the movement of the geometric pattern, thus making it difficult to detect the geometric pattern. Then, if the digital watermark detection apparatus fails to detect the geometric pattern in some of the frames, it becomes difficult to detect the digital watermark, just like when the frames are decimated.
Further, according to the technique disclosed in Japanese Laid-open Patent Publication No. 2006-270215, since the spread pattern is different for each frame, the digital watermark detection apparatus has to estimate the spread pattern for each frame, thus making the watermark detection all the more difficult.